Re: Delaying writes to disk when there's no need

[Andrew Morton]

Nick Piggin <[e-mail]> wrote:
>
> it seems to me that
> doing writeout whenever the disk would otherwise be idle
> (and we have dirty memory to write out) would be a good
> solution.

This is what the recently-removed BDI_read_active flag in backing_dev_info
was supposed to be for.  I let it go because I don't think it's terribly
important and it's time to stop fiddling with the vfs writeout code and it
wasn't right anyway.

Note that 2.5 starts pdflush writeout at 10% of memory dirty.  Or even lower
if there is a lot of mapped memory around.  Whereas 2.4 will start background
writeout at 30% or 40% dirty.  That's a fairly significant tuning change.

The algorithm for utilisation of an idle disk should be, in
balance_dirty_pages():

	if (ps.nr_dirty + ps.nr_writeback < background_thresh) {
		if (time_after(jiffies, bdi->last_read + HZ/100)) {
			if (bdi->write_requests_in_flight < 2) {
				struct writeback_control wbc = {
					.bdi		= bdi,
					.sync_mode	= WB_SYNC_NONE,
					.nr_to_write	= write_chunk,
				};

				writeback_inodes(&wbc);
			}
		}
		return;
	}

Or something like that.  It's pretty close.

It could have pretty bad failure modes.  Short-lived files in /tmp now
perform writeout, which needs to be waited on when those files are removed.